1. Field of the Invention
This invention relates to light extraction from light emitting diodes (LEDs), and more specifically to an LED device layer structure and manufacturing method of the same.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
Gallium nitride (GaN), and its ternary and quaternary compounds incorporating aluminum and indium (AlGaN, InGaN, AlInGaN), have become well established in the fabrication of wide band gap semiconductor light emitting diodes (LEDs) over the last 10 years. These compounds are referred to herein as Group III nitrides, or III-nitrides, or just nitrides, or by the nomenclature (Al,B,Ga,In)N. Devices made from these compounds are typically grown epitaxially using growth techniques including molecular beam epitaxy (MBE), metalorganic chemical vapor deposition (MOCVD), and hydride vapor phase epitaxy (HVPE).
The progress of III-nitride based LED development has brought about great changes in LED technology, with the realization of full-color LED displays, LED traffic signals, white LEDs, and so on. Recently, high-efficiency white LEDs have gained much interest as possible replacement for fluorescent lamps. Nonetheless, more improvement in efficiency is desirable.
There are two principal approaches for improving LED efficiency. The first approach involves increasing the internal quantum efficiency, which is determined by crystal quality and epitaxial layer structure, while the second approach involves increasing the light extraction efficiency. A typical internal quantum efficiency value for c-plane III-nitride blue LEDs is more than 70% [1]. An ultraviolet (UV) LED grown on a low-dislocation GaN substrate has recently exhibited an internal quantum efficiency value of about 80% [2]. There is little room for improvement of these values in the case of c-plane III-nitride LEDs.
On the other hand, there is plenty of room for improving the light extraction efficiency. A number of issues may be addressed in eliminating the internal loss of light, including: using high reflective mirror(s), low reflection surface(s) such as a roughened surface, a high thermal dispersion structure, etc.
The LED structure affects how much light is emitted. In order to increase the light output power from the front side of the LED, conventional LEDs are typically equipped with a mirror placed on the backside of the substrate, or a mirror coating on the lead frame. However, this reflected light is re-absorbed by the active region of the LED, because the photon energy of emitted light is almost same as the band-gap energy of the light emitting materials, such as AlInGaN multi quantum wells (MQWs). Due to this re-absorption of the emitted light by the active region, the net output power or the efficiency of the LED is decreased [3,4].
Therefore, to achieve highly output power efficiency of the LED, device structures in which re-absorption of the light is minimized are desirable. The present invention satisfies this need.